220 



THE HUMAN BODY 



course, or refracted. The amount of refraction depends upon 

 the optical nature of the two media and also upon the angle at 



which the ray strikes the surface 

 of separation. This angle, meas- 

 ured between the incident ray and 

 a line drawn at right angles to 

 the surface between the media, is 

 known as the angle of incidence. 

 The angle which the refracted ray 

 makes with this same perpendicu- 

 lar is the angle of refraction. If 



the ra Y is Passing from a less re- 



FIG. 84. Diagram illustrating the 



refraction of light. A B, surface of fractive to a more refractive me- 



separation between two transparent . . 



media; c D, the perpendicular to the dium it is bent toward the normal; 

 ^"^JSK if passing from a more to a less 

 L th fiX^ m retr,e b d ed r e arif th the refractive medium it is bent away 



second medium is less refractive than from the normal (Fig. 84). The 

 the first. ... , . . J . 



amount of bending is determined 



by the law of refraction which is : the ratio of the sine of the angle 

 of incidence to that of the angle of refraction is always constant for 

 the same two media and for light of the same wave length. 

 This ratio of sines is the index of refraction. It is usually ex- 

 S 



S r ',''' B ^D 



FIG. 85. Diagram illustrating the dispersion of mixed light by a prism. 



pressed for various refractive media with air as the second and 

 less refractive one. 



Dispersion of Mixed Light. The shorter the vibration periods 

 of light-rays the more they are deviated by refraction. Hence 



